Deciphering controls for debris‐flow erosion derived from a LiDAR‐recorded extreme event and a calibrated numerical model (Roßbichelbach, Germany)

Abstract

Debris flows are among the most destructive and hazardous mass movements on steep mountains. An understanding of debris‐flow erosion, entrainment and resulting volumes is a key requirement for modelling debris‐flow propagation and impact, as well as analysing the associated risks. As quantitative controls of erosion and entrainment are not well understood, total volume, runout and impact energies of debris flows are often significantly underestimated. Here, we present an analysis of geomorphic change induced by an erosive debris‐flow event in the German Alps in June 2015. More than 50 terrestrial laser scans of a 1.2 km long mountain torrent recorded geomorphic change in comparison to an airborne laser scan performed in 2007. Errors were calculated using a spatial variable threshold based on the point density of airborne laser scanning and terrestrial laser scanning and the slope of the digital elevation models. Highest erosion rates approach 5.0 m3/m2 (mean 0.6 m3/m2). During the event 9550 ± 1550 m3 was eroded whereas only 650 ± 150 m3 was deposited in the channel. Velocity, flow pressure, momentum and shear stress were calculated using a carefully calibrated RAMMS Debris Flow model including material entrainment. Here we present a linear regression model relating debris‐flow erosion rates to momentum and shear stress with an R2 up to 68%. Channel transitions from bedrock to loose debris sections cause excessive erosion up to 1 m3/m2 due to previously unreleased random kinetic energy now available for erosion. © 2019 John Wiley & Sons, Ltd.

Talus slope geomorphology investigated at multiple time scales from high‐resolution topographic surveys and historical aerial photographs (Sanetsch Pass, Switzerland)

“…This could have increased the magnitude of events linked to a certain return period when the sediment storages in the rockwall are filled. Increased erosion of existing and developing channels could also have played a role, since channel erosion adds considerable amounts of sediment to debris flow volumes, as also shown by other studies (Stoffel, 2010;Dietrich and Krautblatter, 2019). The volumes and associated return periods found for the area (Figure 12c) are in line with other observations from Central and Northern European mountain ranges, despite the uncertainty due to the temporal resolution of the used archival data and the general large variation depending on site-specific conditions (van Steijn, 1996).…”

“…More scanning positions could lead to a better result but are very time consuming; e.g. Dietrich and Krautblatter (2019) needed 53 scans to cover their debris flow area. Volumes of deposition are in turn underestimated by UAV data since the LoD is higher (Table 3) and debris flow deposits are less thick at the outermost parts of their deposits.…”

Modelling future lahars controlled by different volcanic eruption scenarios at Cotopaxi (Ecuador) calibrated with the massively destructive 1877 lahar

“…Although there is no common opinion on whether entrainment of substrate material reduces or enhances flow mobility (Iverson et al, 2011; Mangeney, 2011; Pudasaini & Fischer, 2016), an increase in flow volume has been observed to positively correlate with peak discharge, potential inundation area, runout distance, flow height and velocity (Rickenmann, 1999). The number of numerical models capable of simulating entrainment of mass flows is currently growing (Iverson & Ouyang, 2015), and the performance of the underlying theories is increasingly tested and validated against real mass flow events (Dietrich & Krautblatter, 2019; Frank et al, 2017; Hungr & McDougall, 2009; Hussin et al, 2012). The simulation of entrainment processes either requires the input of user‐specific growth rates or of process‐based erosion rates as a function of velocity (Fagents & Baloga, 2006) or shear stress (Frank et al, 2015; Iverson, 2012).…”

How memory effects, check dams, and channel geometry control erosion and deposition by debris flows

“…There has been a recent increase in the number of numerical models incorporating erosion 21 – 23 , but the inconsistency in erosion rate equations as a result of a lack of a unified theory still results in a disparity of model outcomes. Much of our understanding of debris-flow erosion stems from theoretical considerations 24 and physical scale experiments 13 , 25 – 28 , while there is a relative scarcity of field data 11 , 12 , 29 – 32 as a result of the infrequent nature of debris flows, the rough terrain in which they occur, and the high time and cost demands of field measurements. Analysis of field data is often hampered by unknown boundary conditions and material properties 11 , 12 , and is often based on local point or cross-section measurements 12 , 31 , single time-steps 32 , and measurements are typically only available for small areas 29 , 30 .…”